tests: add a test for rt_sigaction output

tests: add a test for rt_sigaction output

Since "struct sigaction" varies between architectures, rt_sigaction
decoding sometimes produces incorrect output.  This test is expected
to catch basic rt_sigaction decoding bugs.

Based on a patch proposed by Chris Dearman.

* tests/sigaction.c: New file.
* tests/sigaction.awk: Likewise.
* tests/sigaction.sh: New test.
* tests/Makefile.am (check_PROGRAMS): Add sigaction.
(TESTS): Add sigaction.sh.
(EXTRA_DIST): Add sigaction.awk.
* tests/.gitignore: Add sigaction.